HVAC Ductwork Methods

Flexduct Goes in Basements or Attics

• Flexduct is installed with hangers or braces, metal straps typically nailed to ceiling, attic or floor joists, depending on where the installation is. Flexduct can be installed in basements, attics or walls. In basements, flexduct is typically installed with hangers on floor joists, although gas pipes and other elements can be used as supports. In attics, it typically is laid between or over joists and fastened in place with straps to keep it from moving with air that might blow through.

'Trunk and Branch' Design

• Most flexduct is installed in a "trunk and branch" system, with one large supply duct from the unit close to the center of the house feeding various branches that fork off to outlets near exterior walls. Sections are fastened together with metal sleeves that fit inside the ducts and are secured with heat-resistant tape. Flexduct is fastened to the sheet metal plenums or outlets of the unit. It also is used for large return ducts, which collect "used" air from inside the house and take it back to the unit.

Foamboard Fits Special Shapes

• Rigid foamboard can be used to form ducts to fit spaces too small for flexduct, such as inside interior walls. It can be cut with a utility knife and bent into squares or rectangles of any size to accommodate the space. Sections are joined with heat-resistant tape. Foamboard and flexduct can be combined with special connectors that fit inside the rectangular or oval shapes and are secured with heat-resistant tape.

Special Uses for Sheet Metal

• Sheet metal still is used in some special situations, where ducts must fit into very tight or awkward configurations. Its disadvantage is that it is more expensive than either flexduct or foamboard and costs more to fabricate and install. It also has no exterior insulation or moisture barrier and so is less efficient in moving hot or cold air. Flexduct's big advantage, other than its flexibility, is its shape -- a cylinder moves air more efficiently than a box.